1. Field of the Invention
This invention relates to a sensor and associated circuitry for sensing and measuring the presence of a fluid, permeable medium, fluid carried by a solid medium, or other substance, and to a method for using the sensor and associated circuitry. It also relates to an apparatus and method for performing a switching step when the amount of the fluid, the amount of the permeable medium, or the concentration of the fluid carried by said medium reaches a certain level.
Typical applications of the invention are as follows:
(a) Detecting a quantity of rainfall or dispensed water upon an area of land served by an irrigation system;
(b) Sensing leakage, spillage or overflow of oil within a wall surrounding an oil-storage tank;
(c) Detecting leakage of liquid from a conduit into insulation or another substance; and
(d) Making an approximate appraisal of moisture suspended within a sample of earth.
When a quantity of rainfall, the spillage of oil, or the suspension of moisture in a medium is detected, appropriate circuitry produces an electrical signal to operate an alarm or suitable control equipment as required by the circumstances.
Preferably, the sensor comprises at least one positive-temperature-coefficient (xe2x80x9cPTCxe2x80x9d) device coupled to single or plural signal-processing circuits or sub-assemblies for actuating any desired output apparatus for indication or control.
2. Description of the Prior Art
Attention is invited to my co pending application Ser. No. 09/221,733, filed on Dec. 28, 1998 and entitled xe2x80x9cMethod and Apparatus for Sensing and Measuring Plural Physical Properties, Including Temperature.xe2x80x9d That application discloses and claims an arrangement including at least one xe2x80x9ctabletxe2x80x9d of positive-temperature-coefficient (xe2x80x9cPTCxe2x80x9d) material with a plurality of zones that have some electrical and thermal dependence upon each other. The arrangement permits the measurement of at least one physical property, including temperature.
Typically, PTC material comprises a mixture of barium and/or strontium titanates suitably xe2x80x9cdopedxe2x80x9d with certain trivalent or pentavalent elements that serve to adjust the temperature at which the material reaches its xe2x80x9cCurie point.xe2x80x9d At about the Curie point, a plot of resistance of the material as a function of its temperature becomes very steep as temperature increases further. At still higher temperatures, the plot levels off at a xe2x80x9ckneexe2x80x9d as shown in FIG. 1 of the drawings of that application and of this specification. The characteristics of PTC materials are well described in a publication of Keystone Thermometrics, of St. Mary""s, Pa., a copy of which is made a part of the file of this specification.
An xe2x80x9cAtmospheric Sensorxe2x80x9d employing PTC material is shown and described in U.S. Pat. No. 4,890,494xe2x80x94Osbond et al, which is also entered in the file of this specification. That patent discloses a probe of PTC material for measuring the liquid content of a gas. But Osbond et al do not reveal a sensor, which may comprise a single tablet of PTC material divided into zones, which are nevertheless electrically and thermally dependent upon each other. Nor do Osbond et al disclose a circuit having a time-dependent thermal-resistive response to the initialization of an electric potential.
In view of the distinctions of the present invention over the prior art, I have provided a sensing and measuring circuit that is new in its concept and surprising in its capabilities, while employing a modest amount of hardware.
The sensing circuit in accordance with the present invention is built around a tablet of PTC material to which are bonded, preferably on a first side and a second side thereof respectively, first and second respective layers of ohmic resistive (or conductive) material. While the first such layer is continuous in configuration, the second layer is separated into a first zone and a second zone, which are not in direct electrically-conductive relationship with each other. The first layer is connectable, through switching or other means, to a first source of electric potential xe2x80x9c+V.xe2x80x9d The second zone of the second layer is grounded, or connected to a second source of different electric potential.
The first zone of the second layer is in physical contact with a body which, from its standpoint, is a heat sink. The body may, for instance, comprise a diaphragm on the opposite side of which may be present (or not be present) drops of water or other fluid. The first zone of the second layer may be electrically connected through first and second series-resistor means to ground or a source of different electric potential.
The junction or node between the first and second series-resistor means is coupled to the input of a switching device such as an NPN transistor. The output terminals of the switching device may be connected through a gating device to an alarm, a signal light, a control valve, a motor switch, a meter, or other output device.
Means may be provided for periodically applying the voltage xe2x80x9c+Vxe2x80x9d to the first layer of ohmic material. When voltage xe2x80x9c+Vxe2x80x9d is first applied to the first layer, the temperature of the tablet of the PTC material is low, and the current through it immediately becomes high. But, as the current warms the tablet, portions of it reach the Curie point and sharply increase in resistance. Accordingly, the current flowing through the second zone of the second layer and the portion of the tablet of PTC material proximate thereto will sharply decrease.
The portion of the tablet of PTC material which is in contact with the heat sink and with the first zone of the second layer of ohmic resistive material will warm up more slowly than the portion of the tablet proximate the second zone of the second layer. And if the heat sink includes a diaphragm carrying drops of water, oil, or other liquid of high heat capacity, the portion of the tablet proximate the first zone of the second layer will warm up still more slowly. The warm-up time as seen through the first zone of the second layer, when compared with the warm-up time as seen through the second zone of the second layer, will be substantially greater. Moreover, the time disparity will increase with the concentration of heat-absorbing water or oil or other material (the xe2x80x9cheat sinkxe2x80x9d) on the opposite side of the diaphragm or other structure that is in physical contact with the portion of the tablet of PTC material proximate the first zone of the second layer of ohmic resistive material.
The time disparity may be sensed by means of a comparator having one input terminal connected to the first zone of the second layer of ohmic material while the other input terminal is connected to the second zone of the said second layer. For each sensor and for each type of material comprising the xe2x80x9cheat sinkxe2x80x9d, one may compile a correlation table relating the time disparity to the amount or concentration of material in the heat sink.
Without more, such a correlation table would be accurate only for given bands of operating temperatures of the two portion""s of the PTC material. However, a high degree of temperature compensation may be achieved to insure accuracy and reliability of operation over a considerable range of ambient temperatures of the medium in which the tablet of PTC material is accommodated. This temperature compensation may be realized by loosely coupling to the heat sink the portion of the tablet proximate the second zone of the second layer of ohmic material while maintaining tight thermal coupling between the heat sink and the portion of the tablet proximate the first zone of the second layer. To this end, the tablet and its ohmic layers may be supported by a foam having some thermal conductivity, while the assembly of tablet, ohmic layers and foam is confined within a cup or other structure which isolates the assembly from rapid changes in ambient temperature.